No

Although I've never seen any real accuracy increase when using WAAS/EGNOS. Having GLONASS and GPS helps with faster satellite lock, as you're using more satellites, and helps in potentially poor signal areas for same reason.

Battery Life?

You will most certainly get better position accuracy using all three navigational platforms.
I can't speak specifically for the GPSMap 66st but on other GPS units I've used, enabling WAAS & GLONASS can shorten battery life. If this is a concern, you might consider turning them off in areas with good GPS coverage. I believe this is one of the reasons for providing the on / off option.